1. Field of the Invention
The present invention relates to a nitride semiconductor structure and a semiconductor light emitting device, especially to a nitride semiconductor structure with a hole supply layer, and a semiconductor light emitting device with a better light emitting efficiency due to additional holes provided.
2. Description of Related Art
In recent years, light emitting diodes (LED) have become more important in our daily lives due to their broad applications. LED is going to replace most of lighting devices available now and becoming a solid lighting element for the next generation. It's a trend to develop high energy saving, high efficiency and high power LED. Nitride LED has become one of the most popular optoelectronic semiconductor materials due to the advantages of compact volume, mercury-free, high efficiency and long service life. The wavelength of III-nitride almost covers the wavelength range of visible light so that it is a LED material with great potential.
The Group III nitrides such as InN, GaN, AlN, etc play an important role in optoelectronic semiconductor devices due to their wide band gap. The energy gap ranges from InN with a direct band gap of 0.7 eV to GaN with a direct band gap of 3.4 eV, and even 6.2 eV of AlN and the emission wavelength covers from red, through green blue to deep UV region. As to Group III-Nitride semiconductors used as light emitting devices, a p-n junction structure is required. The p-n junction is formed between a n-type semiconductor layer and a p-type semiconductor layer. The n-type semiconductor layer is formed by a semiconductor layer doped with n-type dopant such as Si, Sn, etc while the p-type semiconductor layer is formed by a semiconductor layer doped with p-type dopant such as Mg. However, Mg easily reacts with hydrogen atoms to form Mg—H complexes. Thus the p-type dopant is unable to act as acceptors and the concentration of holes provided is significantly reduced. Therefore the light emitting devices are unable to work well, and a p-type nitride semiconductor layer having a low resistance is difficult to be produced by conventional techniques.
For example, while producing semiconductor layers made from the p-type nitride (such as GaN), ammonia gas (NH3) is used as a source of nitrogen atoms. During the epitaxial process (such as vapor deposition), ammonia gas is decomposed into nitrogen atoms and hydrogen atoms. The hydrogen atoms react and bond with the p-type dopant (such as Mg) used as acceptors in the above semiconductor layer. Thus the p-type dopant loses its activity and the doped concentration is unable to be increased. Moreover, activation energy (Ea) of Mg in GaN is quite large so that the hole activation is less than 10%. Thus it's difficult to raise the hole concentration of the p-type GaN. In order to get high hole concentration, reduction of the Mg—H bonds is required. Therefore the p-type GaN with a sufficiently low resistance is able to provide a better light emitting efficiency.
Thus there is a room for improvement and a need to provide a novel nitride semiconductor structure and a semiconductor light emitting device including the same that overcome the above shortcomings.